<p>Silver birch (<i>Betula pendula</i>) and hybrid aspen (<i>Populus tremula</i> ×  <i>P. tremuloides</i>) are widely planted tree species on former agricultural lands in Northern and Eastern Europe due to their fast growth, high biomass production and CO₂ sequestration. As few comparative studies on the biodiversity of silver birch and hybrid aspen plantations have been carried out, we aimed to evaluate above- and below-ground diversity in silver birch plantations (SBs) and hybrid aspen plantations (HAs) and associate the trends with environmental variables. For this, we collected data on herbs, ground-dwelling bryophytes, epiphytic bryophytes, lichens, closed forest species and soil fungi from 10 SBs and 10 HAs (&lt; 25&#xa0;years of age) in Estonia. The richness and composition of herbs was similar in SBs and HAs, whereas all other groups differed either in species richness, composition or both. Overall, HAs hosted higher richness of ground-dwelling bryophytes and epiphytic lichens and lower richness of epiphytic bryophytes than SBs, which was associated with differences in litter and light conditions. The richness of closed forest species was similar between HAs and SBs and increased in correlation with the area of surrounding forests, whereas the compositions differed. The HAs hosted higher fungal richness of all soil fungi, as well as ectomycorrhizal, saprotrophic and plant pathogenic fungi and differed compositionally from SBs. Based on our findings, both birch and hybrid aspen plantations provide benefits with respect to biodiversity, as they support compositionally different assemblages of bryophytes, lichens, closed forest species and soil fungi.</p>